1. Field of the Invention
The invention relates to a rotary machine.
2. Description of Related Art
In a conventional rotary machine such as a turbine or a compressor, an impeller, in which a plurality of vanes are provided in a housing, is provided to be capable of rotating about a shaft, and a fluid flowing into the housing passes between the vanes of the impeller and then flows out of the housing. The aforementioned turbine converts a kinetic energy of the fluid flowing through the housing into a rotary motion of the impeller. The aforementioned compressor suctions the fluid into the housing, compresses the fluid, and then discharges the fluid from the housing when the impeller is rotated.
To drive a rotary machine such as a turbine or a compressor efficiently, it is effective to reduce a tip clearance between a part of an inner surface of the housing that opposes the vanes of the impeller and the vanes themselves. It has been proposed for this purpose that the tip clearance between the part of the inner surface of the housing that opposes the vanes of the impeller and the vanes themselves be adjusted to a minimum value by forming an abradable seal layer on the inner surface of the housing and then abrading the layer using the vanes of the rotating impeller.
However, when a corner portion of each vane of the impeller on an outlet side of the housing abrades the abradable seal layer formed on the inner surface of the housing during adjustment of the tip clearance between the vane and the part of the inner surface of the housing that opposes the vane, a step is formed on the abraded part. When a step is formed on the abradable seal layer in this manner, the fluid flowing through the housing between the vanes of the impeller may stop flowing smoothly from the vicinity of the corner portion of the vane on the outlet side of the housing toward the outlet of the housing. As a result, it may be difficult to drive the rotary machine efficiently.
Hence, in Japanese Utility Model Application Publication No. 1-148001 (JP-U-1-148001), as shown in FIG. 6, when an abradable seal layer 52 is formed on an inner surface of a housing 51, a step 55 is formed in advance on the abradable seal layer 52 by causing a part of the abradable seal layer 52 corresponding to a vane 54 of an impeller 53 (a part that opposes the vane 54) to project further toward the vane 54 side than other parts. In this case, when the part of the abradable seal layer 52 that corresponds to the vane 54 is abraded by the vane 54 as the impeller 53 rotates, the step 55 formed on the abradable seal layer 52 by the projecting part is reduced. As a result, when a corner portion 54a of the vane 54 on an outlet side of the housing 51 abrades the abradable seal layer 52 formed on the inner surface of the housing 51, formation of a step on the abraded part can be suppressed.
However, even when the step 55 is formed in advance on the abradable seal layer 52, as in JP-U-1-148001, a part of the abradable seal layer 52 that is abraded by the corner portion 54a of the vane 54 on the outlet side of the housing 51 as the impeller 53 rotates is not always abraded by an amount corresponding to a height of the step 55.
The reason for this is that when the impeller 53 rotates, the impeller 53 may shake due to residual unbalance or the like in the impeller 53 of the rotary machine or dimensional tolerance and wear in components such as a shaft and a bearing for supporting the impeller 53 rotatably. In other words, when shaking (vibration) occurs in the rotating impeller 53, variation occurs in the amount by which the corner portion 54a of the vane 54 abrades the abradable seal layer 52 as the impeller 53 rotates. As a result, either the abradable seal layer 52 is abraded too shallowly by the corner portion 54a of the vane 54 such that the abrading amount is insufficient or the abradable seal layer 52 is abraded too deeply by the corner portion 54a of the vane 54 such that the abrading amount is excessive.
When the abrading amount of the abradable seal layer 52 is insufficient, the abrading amount does not reach the height of the step 55 on the abradable seal layer 52, and therefore the step 55 remains, as shown by a dotted line in FIG. 7A. When the abrading amount of the abradable seal layer 52 is excessive, the abrading amount exceeds the height of the step 55 on the abradable seal layer 52, and therefore a new step 56 is formed on the abradable seal layer 52, as shown by a dotted line in FIG. 7B.
When the abrading amount of the abradable seal layer 52 is insufficient such that the step 55 remains on the layer 52 (the dotted line in FIG. 7A), the step 55 causes a flow passage to widen rapidly in the vicinity of the step 55 when seen from the outlet side of the compressor. As a result, the fluid does not flow smoothly in the vicinity of the step 55, and therefore energy loss occurs in the fluid. When the abrading amount of the abradable seal layer 52 is excessive such that the new step 56 is formed on the layer 52 (the dotted line in FIG. 7B), the step 56 causes the flow passage to narrow rapidly in the vicinity of the step 56. As a result, the fluid does not flow smoothly in the vicinity of the step 56, and therefore energy loss occurs in the fluid. Hence, both when the step 55 remains on the abradable seal layer 52 and when the new step 56 is formed on the layer 52, the steps 55, 56 make efficient driving of the rotary machine difficult.